The invention relates to a method and an apparatus for controlling the composition of a fuel-air mixture supplied to an internal combustion engine. More particularly, the invention relates to a method and an apparatus for generating control signals from the position of the main throttle valve in the induction tube as well as from engine rpm and to use these voltages in generating a control current for actuating a fuel metering device in accordance with the requirements of the engine operation.
Customarily, the composition of the operational mixture of an engine is governed by means of a carburetor in which the venturi cross section defines a local vacuum which determines the amount of fuel delivered to the air on the basis of the pressure difference with respect to atmospheric pressure. In fuel injection systems, the air flow is generally sensed with great precision by means of air flaps and a resulting control variable is used to meter out fuel. It should be noted that both of these air and fuel metering systems can be embodied with the required precision only with substantial effort and cost.
It has also been proposed to use simply available variables, namely the engine speed (rpm) and the throttle valve position to generate a control variable which corresponds to the air flow rate at any moment and which can be used to meter out the required fuel. This method introduces the difficulty that the connection between the fuel quantity per unit time on the one hand and the throttle valve position and engine speed on the other hand is a relatively complicated function. In a known circuit for controlling fuel injection valves, a monostable multivibrator receives pulses of rpm-dependent frequency and the unstable time constant is changed in dependence on a voltage related to the throttle valve position. The unstable time constant in the circuit determines the fuel injection period. This system starts with a linear rpm dependence and is able to provide only a relatively coarse adaptation of the injection time to the characteristic curves of injection timing which are specific to a particular internal combustion engine. In another known system, there is generated a voltage which depends on the rpm and on the position of the gas pedal for adjusting the position of a three dimensional cam via electromechanical transducers. The three dimensional cam which represents the operating characteristics of the engine is then followed by a second mechanical-electrical transducer which generates a suitable control variable for influencing the final control element of a fuel injection pump. If sufficient precision is required, this latter known system is very expensive because the attainable precision is adversely affected by the several transducing steps and by the mechanical following of the cam.